1. Field of the Invention
The present invention relates to an image processing apparatus, a control method of the image processing apparatus, and a storage medium. In particular, the present invention relates to an image processing apparatus which performs image processing for correcting misregistration, a control method of the image processing apparatus, and a storage medium.
2. Description of the Related Art
Recently, there has been an increase in electrophotographic color image forming apparatuses (e.g., a laser printer) employing a tandem method to form images at high speed. The tandem-type color image forming apparatus includes a plurality of developing units and photosensitive drums matching the number of color materials (i.e., toner), and sequentially transfers images of different colors to a conveyance belt or a recording sheet to form a color image. In such a tandem-type color image forming apparatus, each image forming unit (e.g., printer engine) corresponding to each color forms the image of a different color. There is thus a plurality of causes for generating misregistration between each of the color images, and various methods for correcting the misregistration are discussed.
The causes of misregistration include non-uniformity and positional displacement of a lens mounted in a deflection scanning apparatus in the image forming unit which deflect-scans the photosensitive drum with a laser beam. Further, misregistration may be caused by positional displacement in assembly of the deflection scanning apparatus in the image forming apparatus main body. When such cases occur, a scanning line of the laser beam on the photosensitive drum (e.g., an exposure scanning line) becomes tilted or skewed. Since a level of such tilt or skew of the scanning line is different for each color, misregistration is generated as a result.
To correct such misregistration, Japanese Patent Application Laid-Open No. 2004-170755 discusses a technique employing an optical sensor which measures sizes of the tilt and the skew of the scanning line (i.e., the exposure scanning line) in the deflection scanning apparatus. Bitmap image data is then corrected to offset the tilt and the skew, and the image is formed based on the corrected image data. More specifically, the bitmap image is corrected by shifting a portion of the image in a sub-scanning direction. The method discussed in Japanese Patent Application Laid-Open No. 2004-170755 processes the image data to electrically correct the misregistration (i.e., by using software), so that mechanical adjustment or adjustment in assembly of the apparatus becomes unnecessary.
Further, resolution of the image forming unit in the image forming apparatus, such as the printer engine of the laser beam printer, has become higher along with recent improvements in semiconductor integration technology. If the technique discussed in Japanese Patent Application Laid-Open No. 2004-170755 is employed, a data amount (i.e., amount of information) of information necessary for electrically correcting the misregistration (i.e., correction information) increases along with an increase in the resolution of the image forming unit. If misregistration is to be corrected, the correction information for correcting the skew of the laser beam scanning line (i.e., exposure scanning line) is required to be stored in a register.
An amount of line correction information to be stored in the register may be calculated as (a number of correction positions in a main scanning direction)×(position to be shifted in the sub-scanning direction). For example, in case of a printer engine whose resolution is 600 dots per inch (dpi) that the number of correction positions in the main scanning direction is 256 and the position to be shifted in the sub-scanning direction is 16 (i.e., 24) steps. In such a case, capacity of the register required for storing the correction information becomes 256×4=1024 bit.
Further, if the resolution of the printer engine is 2400 dpi, it becomes necessary to perform misregistration correction of higher accuracy. More specifically, if the number of correction positions is four times the above-described case, i.e., 1024, and the positions to be shifted in the sub-scanning direction are 64 (i.e., 26) steps, the capacity of the register for storing the correction information is required to be 1024×6=6144 bit. In other words, the capacity necessary for the register becomes six times that of the case where the resolution of the printer engine is 600 dpi. As a result, a circuit size of the image forming apparatus increases, so that cost increases.